The invention relates to a process for separating solids from phosphoric acid produced by the wet digestion process.
Several processes are known for purifying phosphoric acid produced by the wet process. In the case of phosphoric acid produced by the wet process, in which the phosphate rock is digested by sulphuric acid, very large quantities of dissolved or suspended impurities are contained in the sludge produced, e.g., Al.sub.2 K(PO.sub.4).sub.2 OH.2H.sub.2 O, FeH.sub.3 (PO.sub.4).sub.2.4H.sub.2 O or Fe H (H.sub.2 PO.sub.4).sub.4. As these compounds are often in gel-like form, they are very difficult to separate.
DE-OS 24 36 921, for example, describes a process in which purification is achieved by the addition of calcium compounds, by precipitation of gypsum and by separating the sludge together with the gypsum.
Processes are also known in which flocculants are added to the acid (e.g., U.S. Pat. Nos. 3 644 091 and 3 907 680). Other purification processes are described in DE-OS Nos. 26 35 911 and 26 35 912, processes for removing organic compounds in DE-OS No. 25 22 220 or purification processes using organic solvents in DE-OS No. 24 35 789 or DE-OS No. 25 11 340.
The known processes either have a high energy or equipment demand or the throughput rates of dilute acid required for the separation process are so high as to render the process uneconomical.